Gaseous fueled vehicles may store fuel in one or more pressurized tanks (made from carbon fiber, for example) at maximum pressures of approximately 350 to 700 bar. For example, U.S. Pat. No. 5,127,230, describes one example of a liquid natural gas delivery system where two tanks (primary and secondary) are used. Specifically, a primary tank is selected to deliver the fuel to an engine and an automatic override system is provided whereby if the pressure in the non-selected tank rises above a predetermined level, the operator's tank selection is overridden and gas from the non-selected tank is used until the pressure falls below the predetermined level. This override system eliminates the need to vent the gas to the atmosphere when excessive pressure build up occurs.
However, the inventors herein have recognized that while the above approach may selectively utilize two storage tanks at different pressures to limit over-pressure situations, the system may also result in inefficient use of stored fuel. Specifically, depending on the fuel rail pressure and injection system, once a tank reaches a predetermined pressure at or below the injection pressure (which may be as high as 50 to 100 bar in the case of direct injection), the tank is effectively treated as empty. Thus, any remaining fuel cannot be used to operate the engine and propel the vehicle.
To address the above issues, it may be possible to selectively utilize fuel from a plurality of tanks based on engine demand, rather than, or in addition to, tank conditions such as storage pressure.
In this way, it is possible to have full engine output available, while also more completely using stored fuel. For example, even when one storage pressure falls below a desired injection pressure suitable for all operating conditions, that tank may still be used under selected engine demands (e.g., lower engine demands) while another tank at a higher pressure may be used under other engine demands (e.g., high engine demands) to replace or supplement the tank with the lower pressure. In this way, at any given point, as long as at least one storage tank has sufficient pressure, a full scope of engine output is available while allowing more complete use of stored fuel. Such operation can enable improved vehicle operating ranges (e.g., distance) with a full scope of engine output available for a given cycle.
In one example, fuel from tanks at different pressures may be selected based on specific engine loads. For example, a fuel tank with lower pressure may operate during idling and low power portions of a drive cycle, while a fuel tank with high pressure may operate at higher engine loads. In this way, it is possible to more fully utilized stored fuel and extend vehicle range while still maintaining full power capabilities.